For ease of description, we shall refer to the valve when associated with spray drying of milk.
Spray drying techniques are well known and in these milk is sprayed through nozzles into a chamber where the liquid part of the milk is evaporated away and milk powder either falls to the floor of the chamber and is removed therefrom.
Historically, on completion of operation the spray drying industry simply stopped pumping the liquid through the nozzles and as the pressure dropped, a certain percentage of the liquid simply fell as drops from the nozzles, without being atomized, and this material could well lead to pollution of the product in the chamber.
Check valves have been proposed but in practice have suffered from major disadvantages.
A first such disadvantage is that the valves caused a limitation in liquid flow therethrough and there was a large pressure drop over the valve. This meant that in order to operate a plant to capacity, it was either necessary to use higher pressures or larger diameter pipes and valves to enable the optimum flow through the nozzles.
The second disadvantage was that these valves normally had a soft seal, such as an O-ring and it was found, in practice, that this seal could be destroyed during use which would enable parts of the O-ring to pass to the chamber, again polluting the product.
In fact most of these check valves have had a complex design which requires the use of many seals and multiple sealing points. As a result there are many stagnant points such that milk products can build up and, if overheated, catch fire. Additionally soft seals are prone to failure and consequent leakage. As such check valves are generally welded together any repairs are extremely difficult.
Other check valves have used metal to metal seals which increases the wear rate and as discussed above access for repair is difficult in conventional valves.
An additional disadvantage with previous check valves is that as fluid flow is not necessarily smooth through them and stagnant deposits can form they are difficult to keep clean.